Phosgene (COC12) was identified as a metabolite of CHC13 in rat liver in vivo by trapping it with cysteine as 2-oxothiazolidine-4-carboxylic acid. When the experiment was repeated with a 1:1 mixture of CDC13 and (13C)CHC13, approximately twice as much COC12 was trapped from (13C) CHC13 as from CD13. This finding indicates that a deuterium isotope effect is involved in the formation of COC12. Moreover, it appears that COC12 is responsible, at least in part, for the hepatotoxicity produced by CHC13 because CDC13 is less hepatotoxic than CHC13. We believe that the use of stable isotopes and mass spectroscopy outlined in this report can be used to determine the toxic metabolites of other halocarbon compounds.